I'm on a white 3G (16gb) 2.0.2
My friends is a black 3G (8gb) and it's super fast every time he installs a new app. We went out to eat and I played with his for awhile. Every time, zoom, boom, done! (wtf?)
Mine has been slow to install apps from day one back in July, and still is slow even after several complete factory resets. I recently jailbroke my phone, but it didnt affect the appstore install speed. AppStore apps are still slow slow slow! ! !
Update:
After a crash and failed backup restore, I went ahead and started from scratch. This time I did a restore, and picked treat as new iphone. Before installing ANY other program, I used the new WinPwn 2.5 to jailbreak. (toggled to quickpwn mode) Well guess what, now my appstore apps install lightening quick. Even Cydia apps install super fast. I didnt realize the Cydian delay at the end wasnt normal either. I'm not sure what happened but this is the fastest I've seen apps install for me. I'm thrilled. Make sure you never let itunes do a mass install or any install of programs for that matter and you'll be good to go. I think 2.1 should address these problems though, so yeah.